The magnetic structure of sunspots and starspots

The single most important quantity determining the properties of sunspots and presumably starspots is their magnetic field. First an overview of the magnetic structure of sunspots is given, some of the progress made in recent years is described and some of the unsolved questions are pointed out. Both observational and theoretical aspects are dealt with. Finally, the magnetic structure of starspots is discussed. After presenting the evidence for (and against) their magnetic nature the signature of starspots in Zeeman Doppler images is described. It is pointed out that if the properties of sunspots are extrapolated to giant spots on rapidly rotating stars then the observed signature can be at least qualitatively explained.